Neutrinos are similar to the more familiar electron, with one crucial difference: neutrinos do not carry electric charge. Because neutrinos are electrically neutral, they are not affected by the electromagnetic forces which act on electrons. Neutrinos are affected only by a “weak” sub-atomic force of much shorter range than electromagnetism, and are therefore able to pass through great distances in matter without being affected by it. If neutrinos have mass, they also interact gravitationally with other massive particles, but gravity is by far the weakest of the four known forces.
Three types of neutrinos are known; there is strong evidence that no additional neutrinos exist, unless their properties are unexpectedly very different from the known types. Each type or “flavor” of neutrino is related to a charged particle (which gives the corresponding neutrino its name). Hence, the “electron neutrino” is associated with the electron, and two other neutrinos are associated with heavier versions of the electron called the muon and the tau (elementary particles are frequently labelled with Greek letters, to confuse the layman). The table below lists the known types of neutrinos (and their electrically charged partners).
| Neutrino | ne | nm | nt | |||
| Charged Partner | electron (e) | muon (m) |
tau (t)
|
The history of a particle that appeared to have no charge and no mass is an interesting one. The electron neutrino (a lepton) was first postulated in 1930 by Wolfgang Pauli to explain why the electrons in beta decay were not emitted with the full reaction energy of the nuclear transition. The apparent violation of conservation of energy and momentum was most easily avoided by postulating another particle. Enrico Fermi called the particle a neutrino and developed a theory of beta decay based on it, but it was not experimentally observed until 1956. This elusive particle, with no charge and almost no mass, could penetrate vast thicknesses of material without interaction.
The mean free path of a neutrino in water would be on the order of 10x the distance from the Earth to the Sun. In the standard Big Bang model, the neutrinos left over from the creation of the universe are the most abundant particles in the universe. This remnant neutrino density is put at 100 per cubic centimeter at an effective temperature of 2K (Simpson). The background temperature for neutrinos is lower than that for the microwave background (2.7K) because the neutrino transparency point came earlier. The sun emits vast numbers of neutrinos which can pass through the earth with little or no interaction. This leads to the statement “Solar neutrinos shine down on us during the day, and shine up on us during the night!” . Bahcall’s modeling of the solar neutrino flux led to the prediction of about 5 x 106 neutrinos/cm2s.
